Of the structural units of gas, atoms and/or molecules formed by them as well as ions may be mentioned. A single ion or some other structural unit in the gas can momentarily move with a deviating speed and/or to a direction deviating from the flowing direction and/or speed of the gas itself, but on average, a single ion or some other structural unit of the gas in it, however, moves along with the gas. Also short-lived radicals can occur in the gas. Some molecules of the gas can also form loose clusters with polar molecules so that the bond between them is smaller, compared to the strength of a chemical linkage.
A gas sample is a sample to be taken from gas, estimated to represent the gas, from which the sample is taken, with a certain accuracy. A sample gas is a gas, the composition of the gaseous components of which represents the gas sample. The gas sample can also be an aerosol, in which case, in addition to the gaseous phase of the actual sample gas, there may also be present particulate bodies, in a macroscopic sense small pieces, particles, comprising other phases.
Identifying a gas on the basis of certain properties of its structural units can be performed with electrical methods provided that there is a sufficient amount of the structural units of the gas in the ionised state. At least two techniques are known for identifying ions from flowing gas with electrical methods, the IMS technique and the Drift tube, of which also the name drift technique is used. In the IMS technique, ions are analysed from a gas flow, which travels between such measuring electrodes that form an open aspiration condenser. The aspiration condenser has an electric field, the direction of which is perpendicular to the direction of the flow. The electric field deviates ions from the gas flow onto a plate of the aspiration condenser. The flight time and/or flight range of the ions is measured so that it is possible to separate the mobility of ions.
In the Drift technique, ions move in the electric field from a collection lattice to a measuring electrode, from which the magnitude of electric current is measured as a function of time. The zero point for each measurement is set to the zero point of the lattice pulse to be given to the collection lattice, and the ions to be measured move to the measuring electrode usually through a carrier gas with suitable properties. Due to its principle, separate circulations are generally needed for the sample and the carrier gas in the practical realisation of the Drift technique so that the cell is inevitably of a closed structure, as is also the case with the gas circulation.
An IMS technique is known, in which an open cell according to the simplified schematic diagram shown in FIG. 1 is used in the measurement of the sample gas mobility. The cell has an input at the first end of the analysis chamber 106, the gas sample flow 100 going to which is illustrated by an arrow. The chamber 106 itself is restricted by the plates 102 and 108. The cell has an electrode pair consisting of the electrodes 103 and 104 for detecting the ions 101 in the gas sample flow 100. The electrode 103 is attached to the plate 102 and the electrode 104 to the plate 108. The electrode 103 has a certain potential and the electrode 104 a certain second potential. The potential of the electrode 104 is generally close to the ground potential for placing the electric field 105 between the electrodes 103 and 104 and, on the other hand, for generating the voltage signal to be generated against the ground potential. The cell shown in FIG. 1 operates so that, as the gas ion 101 arrives at the space between the electrodes along with the gas sample flow 100, the electric field 105 interacts with the ion 101, in which case the interaction force causes a change of the travelling direction of the ion 101 and, in a certain case, its aggregation to the plate 104 so that the change of charge caused there by the aggregating ions is detectable as an electric current and changeable, for example, to a voltage signal. In cell solutions according to FIG. 1 for identifying gas on the basis of the mobility spectrum of its ions, an alternating voltage of nominally constant value can be used for providing the electric field 105 changing along with it. In this case, the strength of the electric field 105 can be varied, for example, sinusoidally, and/or several such electrode pairs as the pair formed by the electrodes 103 and 104 is used for analysing the charged particles so that the pairs are also attached to the cell limited by the plates 102 and 108 and mounted sequentially, following one another in the direction of flow so that there is an angle, generally a right angle between a mean velocity vector of the sample gas flow 100 and the directional vector of the electric field 105. For example, ions with certain mobility can then be picked up to the plate 104, and slightly different ions can be picked up to a similar second plate for forming the mobility spectrum, and the sample gas can be identified with the help of it.
Cell geometries are also known, in which an electric current caused by ions is detected by electrodes at opposite ends of the chamber so that the angle between the gas flow and the average direction of travel of the ions is approximately 180°. The gas in the drift chamber of the cell can, be let to drift through the drift chamber, for example, with the help of the flow; in some solutions however, also to the opposite direction from the average movement of the ions under the forces created by the electric field.
In the known technique, the incoming sample is charged substantially immediately, and the ions are let to drift along with the flow passing through the chamber but, on the other hand, according to the direction determined by the electric field; in some cases, also deviating from the direction, nevertheless, towards the current target or a respective electrode 104 for collecting the ions, which can also be located, for example, at the opposite end of the analysis chamber from the sample input arranged for sampling. When hitting such a current target as the electrode 104, the ion causes a change of the electric charge in it, which is interpreted as a current signal and processed into a suitable form with signal processing means.
Charging the gas sample can be performed in many different ways. Radioactive sources, light and corona discharge may be the best known charging techniques as such so that the facts generally known about charging depend on the charging mechanism desired to be used and/or on the purpose of use of the charged material, as has been explained in publications dealing with the known technology.
However, the known state-of-the-art cell structure has drawbacks. One of these is connected to the structure of the condenser formed by the electrodes. In the condenser, the change of potential on the plate 103 can be seen in the measurement made from the plate 104. In addition, variations in air humidity and temperature have a detrimental influence on the properties of the condenser, which makes the processing of the current signals caused by the ions more difficult and thus causes uncertainty in the forming of the mobility spectrum, which makes the identification more difficult.
Known IMS technique is described in a patent publication U.S. Pat. No. 5,455,417, the device according to which is illustrated by the cross-sectional drawing in FIG. 1B: The gas entering from the input 128 is heated in constant temperature with the help of the aluminium part 119, which contains the heater 127 controlling its temperature. The gas is charged with the help of the radioactive source 129, after which the gas advances to the analysis cell 125 having the plate electrode 121 and front electrode 122 and collection electrode 123 for the stepwise adjustment of a certain voltage and thus an electric field between the electrodes 121, 122 and 123, as is explained in the patent publication. By using the electric field in the way mentioned, it has been attempted to make the conventional aspiration condenser in FIG. 1B work in a more perfect manner. Among others, the temperature sensor used in the adjustment of temperature, the gas output 120 and the circuit boards 124 and 126 have been drawn to FIG. 1B, electronic components having been drawn to the surface of the latter circuit board 126.
The patent publication also discloses a method related to the technique, in which a sample including the substance to be analysed, the analyte, is first collected and charged. However, the patent specification mentions that, in this case, the concentration of the analyte has to be sufficiently high in the sample in order to achieve a saturation stage in the charging. The mobility of ions is determined from the charged gas sample. The concentration of the analyte in the sample is determined on the basis of the mobility.
The technique has its drawbacks. The massive aerosol particles advancing to the analysis cell 125 after the accumulator can get through the field formed by the electrodes 121 and 122, and most disadvantageously, cause considerable signal distortion on the analysis electrode 123, especially if and when they can carry a considerable electric charge. Further, the possible presence of aerosol particles in the accumulator can have a detrimental effect on the later stages, such as the mechanical and/or electrical blocking of the next analysis chamber, in which case the operation is made more difficult, and the reliability of the analysis result suffers. The possible re-suspension and/or related contact charging can also detrimentally transfer the charge to a wrong place. Another matter is related with heating. Namely, when transferring from heated sections to colder sections, the changes in temperature can cause phase transitions from gas phase to liquid phase and/or solid phase. In this case, the phenomenon in question is the forming of particles, nucleation, which has several subtypes, depending on the starting points of the particle formation. Especially the ion-induced nucleation triggered by radiation and, for example, the heterogenic nucleation taking place in structural defects on surfaces can in some circumstances cause the formation of particle-shaped material and its aggregation to places detrimental for the identification of ion mobility.
State-of-the-art solutions are further limited by a certain slowness in the changes of voltage so that it is also possible that changes occurring in the sample gas during a single measurement can influence the final result.